This invention relates generally to an electromagnetic flowmeter having an electromagnet provided with a magnetizable core whose permability is temperature-dependent, and more particularly to a circuit arrangement compensating for the effect of temperature on the output signal yielded by the flowmeter.
In an electromagnetic flowmeter, an electromagnetic field is established therein by an electromagnet having an excitation coil wound on a magnetizable core, the field being intercepted by a fluid passing through the flow tube to induce a voltage in a pair of electrodes mounted at diametrically-opposed positions of the tube.
Electromagnetic flowmeters are known which make use of low-frequency, uni-polarity or alternating polarity excitation current pulses. One can, by means of a low-frequency excitation type magnetic flowmeter obtain a flow rate signal having an excellent signal-to-noise ratio; for unwanted magnetic and electrostatic couplings between the excitation coil and the signal lead wires connected to the flow tube electrodes may be substantially reduced by low-frequency excitation of the coil.
The voltage derived from the electrodes of the flowmeter is applied to a measuring circuit to produce a signal voltage U that depends on the strength of the magnetic field B and varies in accordance with the flow rate of the fluid being metered. In order to render signal voltage U accurately proportional to flow rate, one must either maintain the strength of the magnetic field constant, or one must compensate signal voltage U for variations in the strength of the magnetic field.
In order to eliminate unwanted fluctuations from the flow rate signal voltage yielded by a magnetic flowmeter which result from fluctuations in the excitation current, the general practice is to provide a reference voltage-detecting circuit. This circuit is arranged to produce a reference voltage proportional to the excitation current, the ratio of this reference voltage to the flow rate signal voltage being determined by means of a converter or divider. To produce the reference voltage, it is known to interpose a resistor in the excitation circuit, the voltage drop across the resistor being proportional to the excitation current and therefore reflecting fluctuations in this current.
In prior attempts to maintain constant the strength of the magnetic field, use was made of a measuring coil interposed in the field to produce a measuring voltage dependent on field strength, which voltage could be used to correct for variations in strength. But where the excitation current for producing the magnetic field is constituted by square-wave direct-current pulses, then at points coincident with the vertical leading edges of the pulses, voltage surges or spikes are induced in this measuring coil that do not reflect the strength of the magnetic field.
While other expedients have also been used to maintain constant the strength of the magnetic field as by regulating the intensity of the excitation current pulses, it has heretofore not been possible to achieve the desired result, which is to provide an output signal that accurately reflects the flow rate of the fluid being metered.
It has been discovered that the reason why prior efforts have failed to fully correct for deviations in the output signal from a proportional relationship with flow rate is that they did not take into account the effect of temperature on magnetization. This effect stems from the temperature-dependent permeability characteristics of the magnetizable core. While the relationship between the permeability of a magnetizable material and temperature is generally known, consideration has heretofore not been given to this factor in flowmeter circuit arrangements to correct for fluctuations in excitation current or to produce a constant magnetic field independent of these temperature effects.
Thus even when the excitation coil of a flowmeter is supplied with direct-current pulses which are maintained constant, the resultant magnetic field strength, despite this expedient, is still not constant; for it is dependent on the temperature of the core.